Production of dielectric boards

ABSTRACT

Copper substantially free of micro-pores is electrodeposited on a polished surface of a stainless steel, titanium, or chromium-plated steel press plate. The copper layer is then provided with a matte surface of copper of dendritic structure which is subsequently bonded to a dielectric material under the application of heat and pressure in a laminating press. The resulting copper-clad dielectric board separates from the press plate, which can then be re-used.

TECHNICAL FIELD

This invention relates to a process for producing copper-clad dielectricboards and to material for use in such a process.

BACKGROUND ART

The background to the invention is as follows. Copper clad laminates areat present manufactured by taking copper foil produced generally inaccordance with the teaching of U.S. Pat. No. 3,674,656, laying it ontop of one or more sheets of partially cured resin impregnated basematerial, and placing the two materials between press plates on alaminating press. Under heat and pressure the partially cured resinadheres to the copper foil so that, when removed from the press, the twomaterials are firmly bonded together.

Such copper foil as is used in this process is available in unsupportedform in a thickness range of 9 μm upwards to in excess of 105 μm. Sincesuch foil is frequently in excess of 1 meter wide, handling sheets of itcan be difficult and, particularly in thicknesses between 9 and 20 μm, agreat deal of scrap is generated in the laying up process. In order topreserve the surface quality of the laminate great care has to be takento exclude all dust particles from between the surface of the copperfoil and the press plates, which are used to separate the laminates inthe press during manufacture.

In order to facilitate the handling of thin copper foils it has beenproposed to manufacture such materials by continuously depositing suchcopper onto a carrier foil of aluminium of chromium-plated copper andprocesses for so doing are disclosed in U.S. Pat. No. 4,113,576 and U.K.Patent Specification Nos. 1,460,849, 1,458,260, and 1,458,259. Inpractice foils produced by these techniques are costly and unreliableand have found little favour in the industry.

U.S. Pat. No. 3,984,598 describes a process in which a stainless steelpress plate known as a caul plate is coated with a silane as a releaseagent and is then electroplated with copper. The exposed surface of thecopper is then oxidised and treated with a silane as a bonding agent.The copper-clad caul plate is then laminated to resin-impregnated basematerial in a laminating press. After the laminate is removed from thepress, the caul plate is removed from the copper-clad dielectric boardwhich has been produced. The copper coating is found to have a variablethickness of about 5 to 12 μm.

It is notoriously difficult to uniformly electroplate a surface providedwith an organic parting layer such as silane. As a result, the depositwill suffer from porosity. During laminating, the resin will thereforeseep through the pores, both causing adherence to the caul plate andcontaminating the surface of the laminate.

What is desired is a method by which thin copper layers of 3 microns andupwards can be successfully and economically laminated to dielectricbase materials with great reliability.

DISCLOSURE OF THE INVENTION

The present invention provides a process for producing copper-claddielectric boards, comprising the sequential steps of

(a) depositing a layer of copper substantially free of micro-poresdirectly on a polished surface of a flat metallic press plate;

(b) providing the copper layer with a matte surface of copper ofdendritic structure;

(c) bonding the matte surface to a dielectric material while applyingheat and pressure to the press plate and the dielectric material in alaminating press and subsequently allowing the press to cool, the forcesgenerated at the interface of the press plate and the copper layer,owing to the penetration of the dielectric material into the dendriticstructure under pressure and the subsequent cooling of the dielectricmaterial, being sufficient to overcome the adhesion of the copper layerto the polished surface of the press plate and thereby to cause thecopper layer to be detached from the press plate;

(d) removing the resulting copper-clad dielectric board from the pressand separating it from the press plate; and

(e) returning the press plate to step (a) and repeating steps (a) to(d).

One or both of the surfaces of the press plate may be used.

BEST MODE FOR CARRYING OUT THE INVENTION

In the preferred process the press plate is 1.5 to 3 mm thick and ismade of stainless steel, titanium, or chromium-plated steel. The pressplate is polished by an abrasive brush or spray to provide a uniformfinish of a surface roughness not exceeding 0.2 μm (preferably 0.1 μm)centre line average (C.L.A.).

After polishing, all traces of abrasive and products of abrasion areremoved by washing.

The polished press plate is immersed in a copper plating bath verticallydisposed and parallel to a suitable anode where it is rendered cathodic.A current is applied so as to plate on the press plate a fine graincopper deposit substantially free of micro-pores. The so plated pressplate is removed from the bath, washed, and placed in a strong coppersulphate bath, again vertically and parallel to an anode. By controllingthe conditions in this bath a further copper layer is deposited in sucha way as to cause a somewhat coarser crystalline layer to be depositedon the fine grain deposit already present.

Subsequent plating in further copper baths under controlled conditionscan be carried out to create a microcrystalline dendritic structurewhich has a high surface area suitable for bonding to typical dielectricbase materials. When the plating sequence is complete the copper platedpress plate is washed, passivated in weak chromic acid, washed again,and dried.

The plate is then taken to a laminating press and laid on top ofsuitable base material such as epoxy resin impregnated glass cloth. Whenthe laminating press is closed and heat is applied, the resin in thebase material is forced into the microcrystalline dendritic structure ofthe copper. During the subsequent cooling of the resin there is createda sufficient force to disturb the adhesion between the copper and thecarrier plate so that when the press is opened it will be found that thecopper layer is completely detached from the carrier plate and is firmlyadherent to the base. If the plating conditions in the first bath areproperly related to the surface texture of the carrier plate, thedetachment of the copper happens so cleanly that the plate canimmediately be passed through the plating cycle again.

Such a method of making laminates avoids completely the hazard ofreeling and unreeling rolls of copper foil, eliminates the commonproblems of surface defects on finished laminates, and because of thefine crystal deposit of the first layer eliminates the problems ofporosity commonly to be found in electroformed copper foil. Use can bemade of polished caul plates which have previously been used in aconventional laminating process.

The total thickness of copper on the press plate is preferably 3 to 12μm, more preferably about 5 μm.

The first copper layer deposited on the press plate can be very thin,e.g. 1 to 2 μm. It may be deposited by electrolysis, e.g. from a coppercyanide bath or a copper pyrophosphate bath, preferably containing 25 to35 g/l of copper, 150 to 310 g/l of P₂ O₇, 1 to 2 g/l ammonia, andhaving a pH of 8 to 9.

If the first strike of copper is carried out from a near neutral platingbath with high throwing power, and the metal carrier plate has thecorrect surface finish, the dense crystal structure of the first layervirtually guarantees that the foil as eventually plated will be freefrom pinholes or micro-porosity. In conventional foil making technologyporosity on thin foils is a major problem because the copper foil isplated all from the same bath and, in the interests of economicalproduction and so that a matte structure can be achieved, the bath usedis an aqueous copper sulphate solution. Such baths, operated at the highcurrent densities required to achieve economical levels of production,always pose difficulties in maintaining control of the nucleation sitesof the copper at the start of the plating process. Micro-contaminationof the drum surface or the solution can result in intercrystallineporosity which permits resin bleed through if such material islaminated. Rigorous testing is carried out by the foil producer andlaminators so that the high standards required result in high scraplevels in the industry. The production of foil in a multi-stagesheet-by-sheet process as now proposed allows the copper core to beplated at high speed from a similar bath to that used in typical drumfoil processes, but the nucleation site problem is avoided by platingthis layer after a first strike. The near-neutral pyrophosphate bathalso assists in obtaining an oxide free surface on the finishedlaminate.

Even if a few micro-pores are present in the copper plated on the pressplate, resin bleed-through is effectively prevented because there is nospace between the copper and the press plate into which the airentrapped in the micro-pores can escape, so that the resin cannot evenenter the micro-pores.

EXAMPLE

A press plate consisting of a sheet of titanium 2 mm thick was polishedto provide a uniform surface of between 0.1 and 0.2 μm C.L.A. Thepolished sheet was placed in a plating tank containing copper cyanidesolution and plate as described in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Copper Cyanide       3.0-16 g/l                                               Sodium Cyanide       4.5-18 g/l                                               Sodium Carbonate     2.0-4 g/l                                                Rochelle Salt        0-6 g/l                                                  pH                   12-13                                                    Temperature          32-82° C.                                         Current Density      1-7 A/dm.sup.2                                           Time                 3-30 s                                                   Anode Material       Copper or steel                                          ______________________________________                                    

The plated sheet was removed from the bath and thoroughly washed in awarm water spray. The sheet was then placed in a copper sulphate platingsolution, rendered cathodic and plated in conditions as described belowin Table 2.

                  TABLE 2                                                         ______________________________________                                        Copper (as metal)    25-110 g/l                                               Sulphuric acid       60-110 g/l                                               Temperature          45-65° C.                                         Current Density      2-110 A/dm.sup.2                                         Anode Material       Lead Sheet                                               ______________________________________                                    

The plating time depends on the thickness of copper required.

After the foregoing plating step the sheet was transferred to a furthercopper sulphate plating bath and subjected to conditions as follows inTable 3. In determining the precise conditions to be used, it isimportant that copper crystals plated are not of a powdery oxidisedcharacter but are pure metallic copper dendrites firmly adherent to thesurface.

                  TABLE 3                                                         ______________________________________                                        Copper (as metal)    15-45 g/l                                                Sulphuric acid       60-90 g/l                                                Arsenic (as metal)   200-500 mg/l                                             Temperature          18-50° C.                                         Anode material       Lead Sheet                                               Current Density      5-220 A/dm.sup.2                                         ______________________________________                                    

The sheet was placed in this bath disposed parallel and in closeproximity to the lead anode and subjected to a continuous but variablecurrent in a range of time and current densities so as to produce astrongly adherent microcrystalline dendritic deposit of high surfacearea.

The so plated sheet was removed from the plating bath, thoroughlywashed, passitvated in a weak chromic acid solution, washed again, anddried. The total thickness of the plated layer was 12 μm.

This sheet was taken to a laminating press and laid upon 5 sheets ofepoxy impregnated glass cloth of a type commonly used in the productionof copper clad laminates. After the press had been closed and heat andpressure applied in accordance with the requirements of the resinimpregnated base material, the press was allowed to cool and thelaminate removed. It was immediately apparent that the titanium sheethad separated from the copper layer and could be lifted off the laminatethat had been made; it was ready for return to the initial plating bath.

The resultant laminate demonstrated a particularly clean, stain freecopper surface and the copper layer was firmly adherent to the base. Thelaminate was subjected to test procedures typical for the industry andwas found to be satisfactory in every respect.

Instead of the copper cyanide solution specified in Table 1 above, acopper pyrophosphate solution may be used under the followingconditions:

    ______________________________________                                        Plating solution:                                                             Copper (as metal)     30 g/l                                                  Pyrophosphate as P.sub.2 O.sub.7                                                                    180 g/l                                                 Ammonia               1 g/l                                                   pH                    8.6-8.8                                                 Temperature           50-55° C.                                        Current density       2.2-4.3 A/dm.sup.2                                      Anode-cathode gap     7-12 cm                                                 Anode material        copper.                                                 ______________________________________                                    

Potassium hydroxide is used to regulate the pH. The plating time dependson the current density and required thickness (generally 1-2 μm). Duringthe plating process continuous aeration of the anode/cathode interspaceis carried out to prevent the copper deposit from `burning`.

The pH of the bath is regulated continuously to maintain it in the range8.6-8.8. Variations on either side of these levels may result in copperwhich adheres too strongly to the press plate or is porous or both.

The process of the invention described above has clear advantages overthe prior art process represented by U.S. Pat. No. 3,984,598. In theprior art process a silane release agent has to be used to facilitateremoval of the laminate from the caul plate after lamination; even so,it is clear that separation of the laminate from the caul plate does notoccur automatically in the laminating press. The known use of temporary(disposable or re-usable) substrates has always required stripping ofthe foil from the substrate mechanically (i.e. by peeling) or chemically(i.e. by dissolving the substrate). The present invention is a radicaldeparture, in that adhesion between the laminate and the substrate isdestroyed during the laminating process, so that the caul plates andlaminates can be separated in the same way as conventional caul platesand laminates.

In the present process an initial copper layer substantially free ofmicro-pores is surmounted by a copper dendritic structure, therebyachieving a copper foil into which the dielectric material can penetrateso as to produce strong bonding (high peel strength) but which foil isimpermeable to the dielectric material. In contrast, the prior artprocess uses a high current density to achieve a copper layer with arough surface, with the inevitable result that the thin copper layer ismicro-porous; furthermore, it is still found necessary to roughen thesurface further by oxidization. Such a surface provides a much weakerbond than a dendritic structure.

In the prior art process, after removal of the substrate from thelaminate, it is expected that the substrate (caul plate) will stillcontain the release agent as a thin film. However, there is a risk thatthe film will become so thin, perhaps locally, that it will no longerfacilitate removal of the substrate. Therefore, checking of recycledcaul plates would be necessary to ensure that the release layer iscontinuous and undamaged. The release layer will, in general, be moreeasily damaged than a polished metallic surface. It will be difficult todetect imperfections in a release layer, whereas imperfections in apolished surface (such as roughening or scratching) are very easilydetected.

In order to operate economically, a laminator would use not onlycopper-clad caul plates (in the production of laminates with very thincopper, i.e. less than 20 μm) but also ordinary polished caul plateswith self-supporting copper foil (for producing laminates with thickercopper layers). If a release agent is used (as in the prior art process)there is a risk that caul plates with and without a film of releaseagent will be mixed up, whereas the present process can make use ofpolished caul plates which have been used in a conventional laminatingprocess.

What is claimed is:
 1. A process for producing copper-clad dielectricboards, comprising the sequential steps of(a) depositing a substantiallyuninterrupted layer of copper substantially free of micro-pores directlyon a polished surface of a flat metallic press plate; (b) providing thecopper layer with a matte surface of copper of dendritic structure; (c)bonding the matte surface to a dielectric material while applying heatand pressure to the press plate and the dielectric material in alaminating press and subsequently allowing the press to cool, the forcesgenerated at the interface of the press plate and the copper layer,owing to the penetration of the dielectric material into the dendriticstructure under pressure and the subsequent cooling of the dielectricmaterial, being sufficient to overcome the adhesion of the copper layerto the polished surface of the press plate and thereby to cause thecopper layer to be detached from the press plate; (d) removing theresulting copper-clad dielectric board from the press and separating itfrom the press plate; and (e) returning the press plate to step (a) andrepeating steps (a) to (d).
 2. A process as claimed in claim 1, in whichthe press plate is made of stainless steel, titanium, or chromium-platedsteel.
 3. A process as claimed in claim 1, in which the layer of copperinitially deposited on the polished surface is 1 to 2 μm thick.
 4. Aprocess as claimed in claim 1, in which the initial copper layer isdeposited electrolytically from a copper cyanide solution.
 5. A processas claimed in claim 1, in which the initial copper layer is depositedelectrolytically from a copper pyrophosphate solution.
 6. A process asclaimed in claim 1, in which the total thickness of copper on the pressplate surface is 3 to 12 μm.
 7. A flat metallic press plate having apolished surface on which a substantially uninterrupted layer of coppersubstantially free of micro-pores has been deposited directly, thecopper layer having a matte surface of copper of dendritic structurecapable of being bonded to a dielectric material under heat and pressurein a laminating press, the adhesion of the copper layer to the polishedsurface of the press plate being sufficiently low that the forcesgenerated at the interface of the press plate and the copper layer,owing to the penetration of the dielectric material into the dendriticstructure under pressure and the subsequent cooling of the dielectricmaterial, will cause the copper layer to be detached from the pressplate.
 8. A press plate as claimed in claim 7, made of stainless steel,titanium, or chromium-plated steel.
 9. A press plate as claimed in claim7, in which the total thickness of copper on the press plate surface is3 to 12 μm.
 10. A process for producing copper-clad dielectric boards,comprising the sequential steps of polishing a press plate to have asubstantially flat and uniform outer surface of between 0.1 and 0.2microns center line average (C.L.A.); depositing an uninterrupted layerof copper substantially free of micro-pores directly on said polishedsurface; providing the copper layer with a matte surface of copper ofdendritic structure; bonding the matte surface to a dielectric materialwhile applying heat and pressure to the press plate and the dielectricmaterial in a laminating press and subsequently allowing the press tocool, the forces generated at the interface of the press plate and thecopper layer, owing to the penetration of the dielectric material intothe dendritic structure under pressure and the subsequent cooling of thedielectric material, being sufficient to overcome the adhesion of thecopper layer to the polished surface of the press plate and thereby tocause the copper layer to detach from the press plate; and removing thedetached copper-clad dielectric board from the press plate.
 11. Aprocess for producing copper-clad dielectric boards, comprising thesequential steps of depositing an uninterrupted layer of coppersubstantially free of micro-pores directly on a polished surface of aflat metallic press plate; providing the copper layer with a mattesurface of copper of dendritic structure; bonding the matte surface to adielectric material while applying heat and pressure to the press plateand the dielectric material in a laminating press and subsequentlyallowing the press to cool, the forces generated at the interface of thepress plate and the copper layer, owing to the penetration of thedielectric material into the dendritic sturcture under pressure and thesubsequent cooling of the dielectric material, being sufficient toovercome the adhesion of the copper layer to the polished surface of thepress plate and thereby causing the copper layer to detach from thepress plate; and removing the detached copper-clad dielectric board fromthe press plate.
 12. A process for producing copper-clad dielectricboards, comprising the sequential steps of depositing an uninterruptedlayer of copper substantially free of micro-pores directly on a polishedsurface of a flat metallic press plate; providing the copper layer witha matte surface of copper of dendritic structure; bonding the mattesurface to a dielectric material while applying heat and pressure to thepress plate and the dielectric material in a laminating press andsubsequently allowing the press to cool, the forces generated at theinterface of the press plate and the copper layer, owing to thepenetration of the dielectric material into the dendritic structureunder pressure and the subsequent cooling of the dielectric material,being sufficient to overcome the adhesion of the copper layer to thepolished surface of the press plate and thereby to cause the copperlayer to be detached from the press plate; and removing the resultingcopper-clad dielectric board from the press and separating it from thepress plate.